The present invention relates to AB-polybenzoxazole (AB-PBO) monomers and processes for synthesizing them.
AB-polybenzoxazole monomers comprise:
(1) an aromatic group:
(2) an o-amino-hydroxy moiety bonded to said aromatic group, which consists of a primary amine group bonded to said aromatic group and a hydroxy group bonded to said aromatic group in a position ortho to said primary amine group: and
(3) an electron-deficient carbon group linked to said aromatic group.
AB-polybenzoxazole monomers preferably conform with formula I: ##STR1## wherein Q is an electron-deficient carbon group: Ar is an aromatic group: and the amine and hydroxy groups are in ortho position with respect to each other.
The monomer is polymerized by polycondensation in a non-oxidizing solvent acid, such as methanesulfonic acid or polyphosphoric acid, at elevated temperatures, as described in Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,772,678 (Sept. 20, 1988): Wolfe et al., Liquid Crystalline Polymer Compositions. Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985): Evers, Thermoxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988).
The resulting polymers comprise a plurality of mer units which each contain:
(1) an aromatic group: and
(2) an oxazole ring fused to said aromatic group and linked at the 2-carbon to an aromatic group in an adjacent mer unit
The polymers preferably comprise a moiety which conforms to formula II ##STR2## wherein n is a number of repeating units in excess of one.
The o-amino-hydroxy moiety of the AB-PBO monomer is extremely sensitive to air oxidation as a free base, so the monomer is ordinarily stored as an acid salt of hydrogen chloride. The release of hydrogen chloride gas during polymerization causes foaming of the polymerization mixture which can interfere with the reaction. Typically, the monomer is dehydrohalogenated at moderate temperatures and, optionally, reduced pressure, in a solution of polyphosphoric acid having low viscosity, and then phosphorus pentoxide is added to the solution before polymerizing to high molecular weight, as described in U.S. Pat. No. 4,533,693 which is previously incorporated by reference. The dehydrohalogenation step requires time and requires that equipment be made of materials which are inert with respect to hydrogen halide gases. What is needed is a monomer which does not require a dehydrohalogenation step.
The condensation of AB-PBO is a polycondensation reaction. In such a reaction, the purity of the resulting monomer is critical, because impurities containing only one reactive group act as chain-terminating agents which hold down the molecular weight of the resulting polymer. What is needed is a high yield process to synthesize AB-PBO monomer having a high purity.